This invention relates to a reflecting mirror device for use in an optical distance measuring apparatus.
In the field of surveying, an optical distance measuring apparatus has been used for precisely measuring distances by utilizing the phase difference of projected and incident beams of light. Such optical distance measuring apparatus have also been combined in a so-called "total station" with a conventional theodolite or photoelectric rotary encoder to thereby carry out not only the distance measurement but also angular measurements. To measure a distance with the optical distance measuring apparatus or the total station, it is necessary to place a reflecting mirror device at a point to be measured so as to reflect the beam of light emitted from the apparatus back to the apparatus. A corner cube type prism has generally been used as the means for reflecting the beam of incoming light along the same line but in the opposite direction. As well known in the art, a corner cube type prism includes a planar incident and emitting surface and three reflecting surfaces which are perpendicular to each other and inclined with respect to the incident and emitting surface. Supposing that the incident and emitting surface and the reflecting surfaces are replaced by a single planar reflecting surface, such imaginary reflecting surface would be at a distance nl from the position of the incident and emitting surface assuming that l is a distance between the incident and emitting surface and the point of intersection between the reflecting surfaces, and n the refractive index of the prism. It would be apparent for obtaining high measuring accuracy that the imaginary reflecting surface must coincide with a vertical line passing through the point to be measured.
Conventional reflecting mirror devices can be broadly classified into two types, the first being a so-called "zero prism constant" type, in which the imaginary reflecting surface is located vertically above the point to be measured when the mirror device is positioned at a predetermined location with respect to the point to be measured by aligning the weight at the tip of a pendulum on the reflecting mirror device, or viewing the point to be measured by a centering telescope. The second type is that in which the imaginary reflecting surface is offset by a distance from the vertical line passing through the point to be measured when the mirror device is approximately positioned with respect to the point to be measured. The offset distance is referred as the prism constant or "offset value" and the value is 30 mm, for example, it is called "offset value 30". The optical distance measuring apparatus using the reflecting mirror device having such prism constant includes an offset mechanism which corrects the measured value of the distance taking into account the prism constant and then displays the corrected value as the result of measurement. For this purpose, the offset mechanism is set in advance in accordance with a specific value of the prism constant.
Practically, however, reflecting mirror devices adapted to be associated with specific ones of the optical distance measuring apparatus may have different prism constants, so that if it is desired to use an optical distance measuring apparatus with a reflecting mirror device having a prism constant which is different from that with which the particular distance measuring apparatus, the offset value corresponding to the prism constant must be changed by readjusting the offset mechanism. Since the offset mechanism is generally incorporated inside the apparatus, readjustment of the offset value can not be made unless the apparatus is disassembled. In order that the reflecting mirror device be compatible to several apparatus, the offset value of the offset mechanism of the apparatus must be changed for each reflecting mirror device.
It is therefore an object of the present invention to eliminate these problems in the prior art devices. Another object of the present invention is to provide a reflecting mirror device whose prism constant can be changed by adjusting the mounting of a prism reflection member assembly in the mirror device.
The reflector device in accordance with the present invention includes optical reflection means for reflecting a beam of light from an optical distance measuring apparatus, frame means for mounting said optical reflection means, said frame means having at least one first reference abutment portion, holder means for holding said frame means, said holder means having at least two second reference abutment portions, whereby the reflector device has a first prism constant when the first reference abutment portion comes into contact with one of the second reference abutment portions of the holder means and a second prism constant which is different from the first prism constant when the first reference abutment portion comes into contact with the other of said second abutment portions of the holder means.